In modern communication systems implementing error detection mechanisms in combination with feedback signaling features so as to positively and/or negatively acknowledge successful and/or unsuccessful reception of data by the receiving entity, respectively, a communication channel between data receiving entity and data transmission entity has to be available to transmit these acknowledgement signals. Typically, a positive acknowledgement is referred to as an “ACK”, while the negative acknowledgement is referred to as “NAK” or “NACK”. The transmission of acknowledgments is typically handled by so-called retransmission protocols which not only implement the feedback signaling, but also data retransmission in response to the feedback signals.
The most common technique for error detection of non-real time services is based on Automatic Repeat reQuest (ARQ) schemes, which may be combined with Forward Error Correction (FEC), called Hybrid ARQ (abbreviated HARQ). If Cyclic Redundancy Check (CRC) detects an error, the receiver requests the transmitter to send additional bits or a new data packet by sending an NACK or ACK message, respectively. From different existing schemes the stop-and-wait (SAW) and selective-repeat (SR) ARQ are most often used in mobile communication.
In modern digital communication systems, the data units or data packets to be sent may be encoded before transmission. Depending on the bits that are retransmitted three different types of ARQ may be defined.
In HARQ Type I the erroneous data packets received, also called PDUs (Packet Data Unit) are discarded and new copy of that PDU is retransmitted and decoded separately. There is no combining of earlier and later versions of that PDU. Using HARQ Type II the erroneous PDU that needs to be retransmitted is not discarded, but is combined with some incremental redundancy bits provided by the transmitter for subsequent decoding. Retransmitted PDU sometimes have higher coding rates and are combined at the receiver with the stored values. That means that only little redundancy is added in each retransmission.
Finally, HARQ Type III is almost the same packet retransmission scheme as Type II and only differs in that every retransmitted PDU is self-decodable. This implies that the PDU is decodable without the combination with previous PDUs. In case some PDUs are so heavily damaged that almost no information is reusable, self-decodable packets can be advantageously sent.
When employing chase-combining the retransmission packets carry identical symbols. In this case the multiple received packets are combined either by a symbol-by-symbol or by a bit-by-bit basis (see D. Chase: “Code combining: A maximum-likelihood decoding approach for combining an arbitrary number of noisy packets”, IEEE Transactions on Communications, Col. COM-33, pages 385 to 393, May 1985). These combined values are stored in the soft buffers of respective HARQ processes.
HARQ in 3G Systems
In the context of third generation communication systems like UMTS, a HARQ protocol is for example used for High-Speed Downlink Packet Access (HSDPA) as well as High-Speed Packet Uplink Packet Access (HSUPA).
In the following the HARQ protocol for HSUPA is described in more detail. Node B controlled Hybrid ARQ allows for rapid retransmissions of erroneously received data packets on the E-DPDCH (Enhanced Dedicated Physical Data CHannel). The HARQ protocol is terminated at the UE on the terminal side and in the Node Bs at the UTRAN side of the UMTS network. Fast retransmissions between UE and Node B reduce the number of higher layer retransmissions, i.e. by RLC protocol, and the associated delays. Thus the quality perceived by the end user is improved.
A protocol structure with multiple stop-and-wait (SAW) hybrid ARQ processes is used for E-DCH (Enhanced Dedicated CHannel), similar to the scheme employed for the downlink HS-DSCH in HSDPA. An N-channel SAW scheme consists of N parallel HARQ processes, each process working as a stop-and-wait retransmission protocol. It is assumed that UE can only transmit data on a single HARQ process each transmission time interval (TTI).
In FIG. 1 an exemplary N-channel SAW protocol with N=3 HARQ processes is illustrated. UE is transmitting data packet #1 on E-DCH on the uplink to the Node B. The transmission is carried out on the first HARQ process. After propagation delay of the air interface Tprop Node B receives the packet and starts demodulating and decoding. Depending on whether the decoding was successful feedback information in form of ACK or NACK is sent in the downlink (or forward link) to the UE on the E-HICH channel (Enhanced Harq acknowledgement Indicator CHannel). In this example the Node B sends an ACK after TNBprocess, which denotes the time required for decoding and processing the received packet in Node B, to the UE. Based on the ACK/NACK feedback on the downlink the UE decides whether it resends the data packet or transmits a new data packet.
The processing time available for the UE between receiving the Acknowledgement and transmitting the next TTI in the same HARQ process is denoted TUEprocess. In the example UE transmits data packet 4 upon receiving the ACK. The round trip time (RTT) denotes the time between transmission of a data packet in the uplink (or reverse link) and sending a retransmission of that packet or a new data packet upon receiving the ACK/NACK feedback for that packet. To avoid idle periods due to lack of available HARQ processes, it is necessary that the number N of HARQ processes matches to the HARQ round trip time (RTT).
Synchronous HARQ in HSUPA
In HSUPA a synchronous HARQ protocol is used, where retransmissions are sent at a predefined time instance. Essentially, a retransmission is sent a predefined time after a previously sent version of the same packet. The HARQ process number can be derived from the timing, i.e. CFN (Connection Frame Number). Employing a retransmission protocol with synchronous uplink transmissions Node B exactly knows when the retransmissions are sent by UE. Hence, the scheduler in the Node B can reserve the required uplink resources, which enables Node B a more precise control on the uplink interference in the cell.
For E-DCH it was also decided to send the HARQ feedback (ACK/NACK) in a synchronous manner, e.g. after a certain time instant upon having received the E-DCH data packet.
Redundancy Versions and Combining
The two fundamental forms of HARQ are Chase Combining and Incremental Redundancy (IR). In Chase combining, each retransmission repeats the first transmission or part of it. In IR, each retransmission provides new code bits from the mother code to build a lower rate code. While Chase combining is sufficient to make Adaptive Modulation and Coding (AMC) robust, incremental redundancy offers the potential for better performance with high initial and successive code rates, at higher signal-to-noise (SNR) estimation error and forward error correction (FER) operating points (i.e., a greater probability that a transmission beyond the first will be needed), albeit at the cost of additional memory and decoding complexity.
A systematic turbo encoded data packet (e.g. E-DCH data packet) contains the original information bits (systematic bits) and additional parity bits (redundancy). The letter S typically denotes the systematic bits, while the letter P typically denotes the parity bits. In an incremental redundancy scheme there are typically self-decodable and non-self-decodable retransmissions. The usage of non-self decodable retransmissions provides the most gain with incremental redundancy. For E-DCH it was decided that there are 4 different redundancy versions for E-DCH, 2 self-decodable and 2 non-self decodable. The first transmission should always be self-decodable.
FIG. 2 shows an exemplary HARQ IR scheme for E-DCH. In the first transmission only systematic bits S are transmitted from the UE to the Node B. The first retransmission contains the first set of parity bits P1. The parity bits are added to the already received systematic bits in the Node B before decoding. In case the decoding fails, the Node B requests a further retransmission by means of a NACK. In the second retransmission the second set of parity bits P2 is transmitted to the Node B. The third retransmission contains the systematic bits S and the first set of parity bits P1. In the given example the initial transmission and the second retransmission are self-decodable, the first and third retransmissions are non-self decodable.
Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on Wideband Code Division Multiple Access (WCDMA) technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive.
However, knowing that user and operator requirements and expectations will continue to evolve, the 3GPP has begun considering the next major step or evolution of the 3G standard to ensure the long-term competitiveness of 3G. The 3GPP recently launched a Study Item “Evolved UTRA and UTRAN”. The study will investigate means of achieving major leaps in performance in order to improve service provisioning and reduce user and operator costs. It is generally assumed that there will be a convergence toward the use of Internet Protocols (IP), and all future services will be carried on top of IP. Therefore, the focus of the evolution is on enhancements to the packet-switched (PS) domain.
The main objectives of the evolution are to further improve service provisioning and reduce user and operator costs as already mentioned. More specifically, some key performance and capability targets for the long-term evolution are                Significantly higher data rates compared to HSDPA and HSUPA: envisioned target peak data rates of more than 100 Mbps over the downlink and 50 Mbps over the uplink        Improved coverage: high data rates with wide-area coverage        Significantly reduced latency in the user plane in the interest of improving the performance of higher layer protocols (for example, TCP) as well as reducing the delay associated with control plane procedures (for instance, session setup); and        Greater system capacity: threefold capacity compared to current standards.        
One other key requirement of the long-term evolution is to allow for a smooth migration to these technologies.
Uplink Access Scheme for LTE
For uplink transmission, power-efficient user-terminal transmission is necessary to maximize coverage. Single-carrier transmission combined with FDMA and dynamic bandwidth allocation has been chosen as the evolved UTRA uplink transmission scheme. The main reason for the preference for single-carrier transmission is the lower peak-to-average power ratio (PAPR) compared to multi-carrier signals (such as OFDMA), the corresponding improved power-amplifier efficiency and assumed improved coverage (higher data rates for a given terminal peak power). In each time interval, Node B assigns users a unique time/frequency resource for transmitting user data thereby ensuring intra-cell orthogonality. An orthogonal access in the uplink promises increased spectral efficiency by eliminating intra-cell interference. Interference due to multipath propagation is handled at the base station (Node B), aided by insertion of a cyclic prefix in the transmitted signal.
The basic physical resource used for data transmission consists of a frequency resource of size BWgrant during one transmission time interval, e.g. a sub-frame of 0.5 or 1.0 ms, onto which coded information bits are mapped. It should be noted that a sub-frame, also referred to as transmission time interval (TTI), is the smallest time interval for user data transmission. It is however possible to assign a frequency resource BWgrant over a longer time period than one TTI to a user by concatenation of sub-frames.
The frequency resource can either be in a localized or distributed spectrum as illustrated in FIG. 3 and FIG. 4. As can be seen in FIG. 3, localized single-carrier is characterized by the transmitted signal having a continuous spectrum that occupies a part of the total available spectrum. Different symbol rates (corresponding to different data rates) of the transmitted signal imply different bandwidths of a localized single-carrier signal.
On the other hand, as can be seen in FIG. 4, distributed single-carrier is characterized by the transmitted signal having a non-continuous (“comb-shaped”) spectrum that is distributed over system bandwidth. Note that, although the distributed single-carrier signal is distributed over the system bandwidth, the total amount of occupied spectrum is, in essence, the same as that of localized single-carrier. Furthermore, for higher/lower symbol rate, the number of “comb-fingers” is increased/reduced, while the “bandwidth” of each “comb finger” remains the same.
The distributed mode may be implemented in different ways:                A user (codeblock) is allocated on multiple distributed resource blocks        A user (codeblock) is allocated on multiple distributed subcarriers or modulation symbols belonging to resource blocks, where the resource blocks are shared by multiple distributed mode users.        A user (codeblock) is allocated on multiple distributed subcarriers or modulation symbols, which are punctured into a resource block used also for localized mode.        
The transmission in distributed mode is generally used to obtain frequency diversity (in contrast to multi-user diversity for localized mode) and, hence, may be useful in the following cases:                The channel quality to the mobile stations (receivers) of the resource blocks is not known sufficiently well at the base station (transmitter), e.g. due to limited or poor CQI (Channel Quality Indicator) feedback and/or due to outdated CQI feedback (e.g. due to high Doppler frequency).        The data to be transmitted is delay critical and the transmission should be made robust        
At first glance, the spectrum shown in FIG. 4 may give the impression of a multi-carrier signal where each comb-finger corresponds to a “sub-carrier”. However, from the time-domain signal-generation of a distributed single-carrier signal, it should be clear that what is being generated is a true single-carrier signal with a corresponding low peak-to-average power ratio.
The key difference between a distributed single-carrier signal vs. a multi-carrier signal, such as e.g. OFDM, is that, in the former case, each “sub-carrier” or “comb finger” does not carry a single modulation symbol. Instead each “comb-finger” carries information about all modulation symbol. This creates a dependency between the different comb-fingers that leads to the low-PAPR characteristics. It is the same dependency between the “comb fingers” that leads to a need for equalization unless the channel is frequency-non-selective over the entire transmission bandwidth. In contrast, for OFDM equalization is not needed as long as the channel is frequency-non-selective over the sub-carrier bandwidth.
Distributed transmission can provide a larger frequency diversity gain than localized transmission, while localized transmission more easily allows for channel-dependent scheduling. Note that, in many cases the scheduling decision may decide to give the whole bandwidth to a single UE to achieve high data rates.
It should be noted, that multiplexing of localized mode and distributed mode within a sub-frame is possible, where the amount of resources (RBs) allocated to localized mode and distributed mode may be fixed, semi-static (constant for tens/hundreds of sub-frames) or even dynamic (different from sub-frame to sub-frame).
In localized mode as well as in distributed mode in—a given sub-frame—one or multiple data blocks (which are inter alia referred to as transport-blocks) may be allocated separately to the same user (mobile station) on different resource blocks, which may or may not belong to the same service or Automatic Repeat reQuest (ARQ) process. Logically, this can be understood as allocating different users.
Uplink Scheduling Scheme
The uplink scheme should allow for scheduled (Node B controlled) access.
In case of scheduled access the UE is dynamically allocated a certain frequency resource for a certain time (i.e. a time/frequency resource) for uplink data transmission. More specifically the scheduler determines                which UE(s) that is (are) allowed to transmit,        which physical channel resources (frequency, subband, sub-carrier, resource block),        for how long the resources may be used (number of sub-frames, number of TTIs)        Transport format (Modulation Coding Scheme (MCS)+transport block size) to be used by the mobile terminal for transmission        
The allocation information is signaled to the UE via a scheduling grant, sent on the downlink control channel. For simplicity reasons this channel is called Grant Channel in the following. A scheduling grant message contains at least information on which part of the frequency band the UE is allowed to use, whether localized or distributed spectrum should be used, the validity period of the grant, and the maximum data rate. The shortest validity period is one sub-frame. Additional information may also be included in the grant message, depending on the selected scheme.
Uplink data transmissions are only allowed to use the time-frequency resources assigned to the UE through the scheduling grant. The transmission of new data can only occur with a scheduling grant. Unlike in HSUPA, where each UE is always allocated a dedicated channel there is only one uplink data channel shared by multiple users (UL SCH) for data transmissions. Furthermore, there is only one mode of operation for the uplink data access in LTE, the above described scheduled access, i.e. unlike in HSUPA where both scheduled and autonomous transmissions are possible.
To request resources, the UE transmits a resource request message to the Node B. This resources request message could for example contain information on the amount of data to transmit, the power status of the UE and some Quality of Services (QoS) related information. This information, which will be referred to as scheduling information, allows Node B to make an appropriate resource allocation. In a multiple access or shared data channel communication system there may co-exist several resources at the same time which may be granted independently to several users or services. Therefore there may exist also multiple grants in the same time slot. Such grants are preferably transmitted using a shared or dedicated control channel, for simplicity called “Grant Channel”. Consequently there may also exist multiple ACK/NACK signals or channels that are to be transmitted within the same time slot.
Downlink Access Scheme for LTE
FIG. 5 shows a packet-scheduling system on a shared downlink channel for systems with a single shared data channel. A time slot (also referred to as a subframe or PHY Frame herein) reflects the smallest interval at which the scheduler (e.g. the Physical Layer or MAC Layer Scheduler) performs the dynamic resource allocation (DRA). Further, typically the smallest unit of radio resources (referred to as a resource block herein), which can be allocated in OFDM systems, is defined by one time slot in time domain and by one subcarrier/subband/resource block in the frequency domain. Similarly, in a CDMA system this smallest unit of radio resources is defined by a time slot in the time domain and a code in the code domain. In OFCDMA or MC-CDMA systems, this smallest unit is defined by one time slot in time domain, by one subcarrier/subband/resource block in the frequency domain and one code in the code domain. Note that dynamic resource allocation may be performed in time domain and in code/frequency domain.
The main benefits of packet-scheduling are the multi-user diversity gain by time domain scheduling (TDS) and dynamic user rate adaption.
Assuming that the channel conditions of the users change over time due to fast (and slow) fading, at a given time instant the scheduler can assign available resources (codes in case of CDMA, subcarriers/subbands in case of OFDMA) to users having good channel conditions in time domain scheduling. For explanatory reasons, the following sections will mainly concentrate on OFDMA downlink transmission.
Specifics of DRA and Shared Downlink Channel Transmission in OFDMA
Additionally to exploiting multi-user diversity in time domain by TDS, in OFDMA multi-user diversity can also be exploited in frequency domain by FDS (Frequency Domain Scheduling). This is because the OFDM signal is in frequency domain constructed out of multiple narrowband subcarriers (typically grouped into subbands), which can be assigned dynamically to different users. By this, the frequency selective channel properties due to multi-path propagation can be exploited to schedule users on frequencies (subcarriers/subbands/resource blocks) on which they have a good channel quality (multi-user diversity in frequency domain).
For practical reasons in an OFDMA system the bandwidth is divided into multiple subbands or resource blocks, which consist out of multiple subcarriers. I.e. the smallest unit on which a user may be allocated would have a bandwidth of one subband and a duration of one time slot (which may correspond to multiple OFDM symbols), which is denoted as a RE (Resource Element). Typically a subband consists of consecutive subcarriers, however in some case it is desired to form a subband out of distributed non-consecutive subcarriers. A scheduler may also allocate a user over multiple consecutive or non-consecutive subbands and/or time slots.
E.g. for the 3GPP Long Term Evolution (see 3GPP TR 25.814: “Physical Layer Aspects for Evolved UTRA”, Release 7, v. 1.2.2, March 2006—available at http://www.3gpp.org), a 10 MHz system may consist out of 600 subcarriers with a subcarrier spacing of 15 kHz, which may then be grouped into 50 subbands (a 12 subcarriers) with each subband or resource block occupying a bandwidth of 180 kHz. Assuming, that a time slot has a duration of 1.0 ms, then a resource element would span over 180 kHz and 1.0 ms.
In order to exploit multi-user diversity and to achieve scheduling gain in frequency domain, the data for a given user should be allocated on resource elements on which the users have a good channel condition. Typically, those resource elements are close to each other and therefore, this transmission mode is in also denoted as localized mode (LM). An example for a localized mode channel structure has been discussed with respect to FIG. 3 above. In this example neighboring resource elements are assigned to four mobile stations (MS1 to MS4) in the time domain and frequency domain. For exemplary purposes it is also assumed that in the “gaps” between the different resource elements in the time domain, Layer 1 and/or Layer 2 control signaling is transmitted.
In localized mode as well as in distributed mode in a given time slot multiple codeblocks (which are referred to as transport-blocks in 3GPP terminology) may be allocated separately to the same user on different resource elements, which may or may not belong to the same service or ARQ process. Logically, this can be understood as allocating different users.
Shared Channel Related Control Signaling
In order to inform the scheduled users about their allocation status, transmission format and data related parameters Layer 1 and Layer 2 control signaling is typically transmitted along with one or multiple shared data channels (SDCHs).
In 3GPP HSDPA (CDMA) the Layer 1/Layer 2 control signaling is transmitted on multiple shared control channels (SCCHs) on a transmission time interval (TTI)-basis (a TTI may thereby correspond to a time slot in its length). Each transmitted shared control channel carries for example information for one scheduled user, such as channelization-code-set, modulation scheme, transport-block size information, redundancy and constellation version, HARQ process information, new data indicator (similar to a HARQ sequence number) and user identity (see e.g. 3GPP TS 25.212: “Multiplexing and channel coding (FDD)”, Release 7, v. 7.0.0, March 2006, available at http://www.3gpp.org).
Generally, the information sent via Layer 1/Layer 2 control signaling may be separated into two categories, shared control information (SCI) and dedicated control information (DCI). The shared control information part of the Layer 1/Layer 2 control signaling contains information related to the resource allocation and it should therefore be possible for all users to decode the shared control information. It typically contains the following information:                User identity        RE allocation information        
Depending on the setup of other channels and the setup of the dedicated control information, the shared control information may additionally contain information such as ACK/NACK for uplink transmission, MIMO related information, uplink scheduling information, information on the dedicated control information (resource, MCS, etc.).
The dedicated control information part of the Layer 1/Layer 2 control signaling contains information related to the transmission format and to the transmitted data to a specific scheduled user. I.e. the dedicated control information needs only to be decoded by the scheduled user. The dedicated control information typically contains information on the transmission format:                Modulation scheme        Transport-block size (or coding rate)        
Depending on the overall channel configuration, depending on the shared control information format and depending on the HARQ setup it may additionally contain information such as HARQ related information (e.g. HARQ process information, redundancy and constellation version, new data indicator), MIMO related information.
Layer 1/Layer 2 control signaling may be transmitted in various formats. One option is joint encoding of shared control information and dedicated control information. Thereby, shared control information and dedicated control information for multiple users (codeblocks) are encoded jointly or the shared control information and dedicated control information are encoded jointly for a single user (codeblock) and are transmitted separately per user (codeblock).
Another option is the separate encoding of shared control information and dedicated control information. Thereby, the shared control information for multiple users (codeblocks) are encoded jointly or the shared control information is encoded per user (codeblocks). Similarly, the dedicated control information for multiple users (codeblocks) is encoded jointly or the dedicated control information is encoded per user (codeblocks).
In case of having multiple shared control information codeblocks (each shared control information codeblock may contain shared control information for multiple users), the shared control information codeblocks may be transmitted with different power, modulation, coding schemes and/or code rates.
From a logical point of view, the Layer 1/Layer 2 control signaling contained out of shared control information and dedicated control information may be seen e.g. as follows:                A single (shared) control channel with two parts (shared control information and dedicated control information)        A single (shared) control channel (carrying only shared control information), where the dedicated control information is not considered a separate control channel, but part of the shared data channel, i.e. mapped together with the data (same RE)        Two separate control channels (shared control information, dedicated control information)        Multiple separate control channels, e.g.:                    Single shared control channel carrying shared control information and multiple dedicated control channels carrying dedicated control information            Multiple shared control channels carrying shared control information and multiple dedicated control channels carrying dedicated control information            Multiple shared control channels carrying shared control information, where the dedicated control information is not a separate control channel, but part of the shared data channel, i.e. mapped together with the data (same RE)                        
Typically, both the shared control information and the dedicated control information is mapped separately from the shared data channel into the physical resources, which may also be called shared control channel. Alternatively, the dedicated control information may be mapped into the resources allocated for the shared data channel, such that a part of individual shared resource elements is reserved for dedicated control information.
Link Adaptation (LA) Techniques
In order to efficiently utilize the benefits from scheduling in uplink and downlink, respectively, usually it is combined with fast LA (Link Adaptation) techniques such as AMC (Adaptive Modulation and Coding) and ARQ (Automatic Repeat reQuest). Additionally, fast and/or slow power control may be applied.
Employing adaptive modulation and coding (AMC), the data-rate per codeblock (in case a codeblock spans over multiple resource elements, the AMC may alternatively be performed per resource elements) for a scheduled user is adapted dynamically to the instantaneous channel quality of the respective allocated resource by changing the MCS (Modulation and Coding Scheme). Naturally, this requires a channel quality estimate at the transmitter for the link to the respective receiver.
Identification of the Entity to Receive ACK/NACK Signaling
In order to positively identify which ACK/NACK signal corresponds to which data packet, one possibility is to attach an identifier to the ACK/NACK. The identifier is preferably a User Equipment Identifier (UE-ID) or a Group Identifier (G-ID). Since data packets are usually transmitted to a single UE or group of UEs, the UE-ID or G-ID may be used to unambiguously define the target UEs and therefore the corresponding data packet.
However, the attachment of an identifier like a UE-ID or G-ID to the ACK/NACK message is potentially inefficient, because a single ACK/NACK signal commonly consists of only one bit, while an identifier may contain up to 16 bits or more, depending on the communication system. Therefore it is desirable to save as much of this overhead as possible in order to increase the spectral efficiency.
In 3GPP TSG RAN WG1 Tdoc R1-063326 “ACK/NACK Signal Structure in E-UTRA Downlink” by NTT DoCoMo, Fujitsu, Mitsubishi Electric, NEC, Sharp and Toshiba Corporation (submitted for a meeting in Riga, Latvia, Nov. 6 to 10, 2006, available at http://www.3gpp.org and incorporated herein by reference), it is proposed to link the index of the ACK/NACK channel to the index of the L1/L2 control channel. A flow chart illustrating the proposed scheme is illustrated in FIG. 6.
Those skilled in the art will realize that said L1/L2 control channel is an instance of said grant channel. This obviates the transmission of an additional identifier for the ACK/NACK message. Assuming the grant is transmitted on grant channel #x, then the ACK/NACK message corresponding to the data transmission granted by grant channel #x is transmitted using ACK/NACK resource #x. Generally the ACK/NACK resource may be one of a frequency, time, code, or antenna resource. In an abstract way it may be seen as an information field, e.g. a bit field, contained within a generalized control signal structure. In the context of OFDM or other multi-carrier communication systems the division in frequency domain may be expressed as a sub-carrier. According to TDoc R1-063326, the UE ID-less transmission is based on the one-to-one relationship between the index of the downlink L1/L2 control channel for uplink radio resource assignment and the index of ACK/NACK radio resources (e.g. index of sub-carrier sets for Frequency Division Multiplex (FDM) or code index for Code Division Multiplex (CDM)).
One potential drawback of the solution according to Tdoc R1-063326 may introduce ACK/NACK collisions. In a system where retransmissions of a packet are not scheduled, i.e. where no grant is transmitted for retransmissions, there is the possibility that a retransmission of a packet where the first transmission was granted on grant channel #x at time t occurs together with a new packet possibly pertaining to another user which is granted on grant channel #x at time t+Trtt. According to Tdoc R06-3326, the ACK/NACK signal for those two packets (one new transmission and one retransmission) should both use ACK/NACK resource #x at time t+2Trtt, causing a conflict which may introduce signaling errors, additional delay, or higher layer protocol violations. This potential drawback is illustrated for exemplary purposes in FIG. 7.